1. Technical Field
The present invention relates to a light-emitting element, a light-emitting device, a display device, and an electronic apparatus.
2. Related Art
An organic electroluminescence element (a so-called organic EL element) is a light-emitting element having a structure in which at least one luminescent organic layer is interposed between an anode and a cathode as a light-emitting layer. In such a light-emitting element, by applying an electric field between the cathode and the anode, electrons are injected into the light-emitting layer from the cathode side and holes are injected into the light-emitting layer from the anode side. The electrons and the holes recombine in the light-emitting layer thereby generating an exciton. When the exciton returns to the ground state, a corresponding amount of energy is emitted as light.
In such a light-emitting element, in general, a hole injection layer and a hole transport layer are provided between the anode and the light-emitting layer in order to enhance a hole-injecting property or a hole-transporting property (for example, refer to Japanese Patent No. 3654909).
In addition, in the light-emitting element, by adjusting the levels of the HOMO (highest occupied molecular orbital) and the LUMO (lowest unoccupied molecular orbital) of the hole transport layer, the hole transport layer blocks the electrons from the cathode side (the light-emitting layer side), and the electrons and the holes are confined in the light-emitting layer, such that enhancement of a light-emission efficiency is achieved.
However, in the light-emitting element according to the related art, there are problems in that the hole transport layer cannot sufficiently block electrons from the cathode side, and the hole transport layer or the hole injection layer is deteriorated by the electrons passing through the hole transport layer with long-term use. These problems become significant since a function of blocking the electrons in the hole transport layer (electron-blocking effect) is degraded by a band bending effect as a current density is increased. Therefore, a long lifetime of a light-emitting element with high luminance which requires a high-density current cannot be achieved.
In addition, in order to increase the electron-blocking effect, use of a material having an energy difference between the HOMO and LUMO for the hole transport layer is considered. However, this is difficult in practice.